Chiang has proven when given breathing room (Northern Expeditions), he can smush the warlords easily enough and co-opt the survivors. Once the Japanese are defeated, and Mao is pre-ordained to lose (via OP fiat), it's just a matter of time for Chiang to properly unify the country under the KMT banner and kill/exile/cow the remaining warlords and major factions that would oppose future reforms.
I'd give Chiang 5 more years post Reunification Victory, to get shit in order (most important is squaring the army away, centralizing army pay, thus making sure its loyal), and then start on some major reforms. Once a coupla major reforms are out of the way, he'll start liberalizing the government as popular pressures from the people grow as a way to mollify them. With pressure from the US (giving ample gibs) to act a certain way, i dont see Chiang unleashing his loyalist armies to crackdown on the middle class/intelligentsia who likely would be the 1st coupla waves of protesters. I can see him exile the worst of the ringleaders to the US if they prove to be too much of a pain in the ass to local stability. I can see Chiang slowly give way to Democracy as he gets older and sees his nation becoming more cohesive, stronger, and able to stand on its own. China will then go into a full blown Democracy with his son stepping down as military head and assuming a major role in civilian politics, many other political generals and their families will follow suit, the more apolitical generals will stay on, thus mirroring a tradition like their close ally the US.
Heck after Reunification, if Chiang was feeling particularly magnaminous instead of mass executions of the Communist rank and file (key leadership of course are getting shot/hanged) he'd give em a choice to renounce the CCP and swear oath of loyalty to China, or exile to the USSR. This could show how merciful Chiang is, thus potentially making future fanatical insurgencies less likely, while also giving the Soviets a sop to show how theyre not totally hostile to it and can come to a deal in the future. Basically once China becomes stronger they'll take back the exiles as a way to build up relations. Soviets in the meantime can use the exiled commies to build up their Far Eastern possessions, to defend against any future threat on those borders (why would China give us manpower to fight against them?). From Chiang's PoV best to get these troublemakers out of the way for now, without gaining massive ire from the USSR (mass killing commies) and USA (mass killing POWs). Extra LOLs for allowing a Socialist party to spring up later on when China is democratizing. When re-accepting exiles and their families in the future they're to join the Socialist party and any craziness/insurgency led by these exiles will make the Socialists look REAL bad discrediting them. Thus the Socialists would doing their utmost to make sure these communists dont do anything crazy and if THEY purge the commies on their own.... TROLOLOLOLO
With US support the Chinese economy will rapidly rebuild after the turmoil of the 1st coupla major reforms (land reform being the doozy of course), and China will slowly lurch ahead and be a coupla ticks below OTL SKorea in GDP. Sadly for the rest of the Asian continent, their own developments will be retarded compared to OTL due to US pouring their efforts into propping a friendly China up instead of containing it. Japan, SKorea both taking the biggest shellackings. SE Asia might not be as majorly negatively impacted due to being less of a hellscape via constant wars. Would be interesting to see how long France will try to hold Indo-China though. I dont see KMT China interfering there much as they want to keep the Euros as friendly to them as possible to encourage future investments.
I personally see China well on its way to being a Superpower able to challenge the US economically by 2000s.